The present invention relates to a transducer and apparatus for measuring corneal thickness.
Accurate measurement of corneal thickness has become more important with the introduction of a new operation for myopia called "radial keratotomy". In this operation a number of radial cuts are made in the cornea. The healed cuts result in scarring which in turn increases corneal refraction tending to reduce myopia. Since the cornea is very thin (approximately 0.6 mm) great care must be taken during this operation to insure that the cornea is not pierced. However, since the ultimate success of the operation improves with the deepness of the cut made, there is a desire to make the cut as deep as possible. To do this requires accurate knowledge of the thickness of the cornea for each patient It is also desirable to be able to map the cornea, since its thickness varies over its surface to some extent.
The use of ultrasonics in eye measurements is known and shown, for example, in U.S. Pat. No. 4,154,114, issued to L. Katz et al on May 15, 1979 and entitled "Biometric Measuring Device". Also, the use of small transducer at the end of a hand-held probe for determining the thickness of the cornea by conventional A-scan ultrasound methods is known in the art. The transducer is placed against the cornea. Echoes are received from the anterior and posterior surfaces of the cornea, and an electronic circuit measures the distance between the two faces by conventional techniques. To insure that the face of the transducer is perpendicular to the radius of curvature of the cornea, the electronic circuit prevents measurement unless the echos from the two faces meet some predetermined criteria concerning relative size. Thus, if the transducer is angulated by too great an amount, there will be no or only a small echo from the posterior surface and the system will indicate that the reading is not accurate. In spite of the foregoing, use of these systems do not give repeatable results. Thus, the expected and required accuracy is not achieved.
The principal reason for the aforementioned problem is that with the hand-held probe and the type of transducer used with said probe, the pressure applied to the cornea will almost always be so great as to deform the cornea enough to make the measurement useless. This excess pressure is due, in large part, to the flat configuration of the transducer which causes the user to apply excessive pressure in an attempt to obtain better contact with the curved surface of the cornea. It is also virtually impossible for users of current hand-held probes and simple transducers to maintain the holder and transducer perpendicular to the cornea. The cornea is very soft, and there will almost always be some angulation with present systems. Experience has shown that the aforementioned technique or comparing echoes from the two surfaces of the cornea is not sufficiently accurate. That is, with angulation portions of the cornea will be compressed and therefore indicate lesser than actual distances or depth of cornea. Further, the excess pressure results from the application, by the operator, of too much pressure on the probe in an effort to maintain the transducer in proper position on the eye.
Accordingly, it is an object of this invention to provide a transducer and apparatus for measuring corneal thickness which are capable of obtaining repeatable and accurate measurements.
It is a further purpose of this invention to provide such a transducer and apparatus which prevent the application of excess pressure to the cornea.
Another object of this invention is the provision of a transducer which can be easily maintained at the proper position on an eye without requiring the operator to apply excessive pressure on the transducer probe.
An additional object of the present invention is to provide an apparatus which insures that the measurement is taken with the transducer oriented perpendicular to the surface of the cornea.